Abstract: In nonzero-offset VSP surveys, the source-well distance is an important parameter. Under certain geological conditions,excessively larger source-well distance may cause refractions along interfaces to reach geophones earlier than direct arrivals. Based on model analysis and a set of field z-component data obtained from a nonzero-offset VSP in well 5 of Tazhong,this paper investigates the influence of excessively large source-well distance on anomalous first arrivals in VSP data and points out the way the refraction effect in nonzreo-offset VSP is removed. In addition. a method to determine the maximum souce-well distance is presented.